A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Lithographic apparatus usually comprise components which act as undesired heat sources. An example of such a component is an electromagnetic actuator in which a current interacts with a magnetic field (e.g. produced by permanent magnets) to generate actuation forces. The current is carried by a conductor, i.e. a coil, which has a certain electrical resistance. Due to this resistance energy will be dissipated and heat is generated as an undesired by-product of the actuation forces.
The undesired heat may be transported, i.e. leak, to other parts of the lithographic apparatus such as optical elements or substrate tables and cause deformations there which may have an impact on overlay and position accuracy.
Therefore, the heat sources are usually cooled to remove the generated heat and thus prevent heat transport to other parts of the lithographic apparatus. However, the maximum cooling capacity may be restricted by the available space. This is especially the case for electromagnetic actuators in which the efficiency is dependent on the distance between coil and permanent magnet, and wherein the smaller the distance the more efficient the actuator is. An efficient electromagnetic actuator thus leaves little space for a cooling system. As a result, the available cooling capacity may be less than the amount of heat generated, so that an excess of heat may still leak to other parts and impact overlay and position accuracy.